Coated metals and solutions and process for making the same



United States COATED METALS AND SGLUTIONS AND PROCESS FOR MAKING THESAME Donald S. Andrade, Parma' Heights, Ohio, assignor to Parker RustProof Company, Detroit, Mich, a corporation of Michigan No Drawing.Application November 20, 1958 Serial No. 775,115

8 Claims. Cl. 117-427 The present invention relates generally to metals,metallic articles and surfaces having an adherent, protective coatingthereon. More particularly, the invention relates to metallic articles,the surfaces of which are provided with an integral. coating comprisingthe heat reaction product of tertiary butyl alcohol and chromic acid,which coating is tightly adherent, protects the metal against corrosionand improves the adherence of paints, enamels, lacquers, varnishes andother protective finishes thereto, and to the method of formingsuchcoatings.

Heretofore, metal articles and metal surfaces have been coated withinsoluble reacted chemical coatings such as phosphates, oxalates,sulfides, oxides, etc, to protect the surface against corrosion, asdrawing aids and to improve the adherence of paints and other finishingmaterials. For best results, phosphate coatings have been rinsed indilute aqueous chromic acid solutions and dried prior to painting.

An object of the present invention is to provide a bare metal surfacewith a coating which makes the surface resistant to corrosion and towhich paint and other finishing materials will tenaciously adhere, allwithout provision of further coatings or treatments.

Still another object is to provide a process of only two steps whichprovides a corrosion-resistant coating on bare metal to which paint andother materials will tenaciously adhere.

Yet another object is to. provide a solution for accomplishing the aboveand other objects and which is less expensive than known coatingmaterials.

The above and other objects and advantages, according to this invention,are obtained by applying to a bare metallic surface a solutioncontaining as the essential coating-forming ingredients tertiary butylalcohol and chromic acid, and forming a coating on that surface whichcomprises the heat reaction product of the tertiary butyl alcohol andthe chromic acid. A metal article, the surface of which is coated withan integrally bound coating comprising the heat reaction product oftertiary butyl alcohol and chromic acid, constitutes the improvedarticle of this invention.

The metal surfaces which can be protected by the coatings of thisinvention include the surfaces of all of the industrial metals includingiron, steel, stainless steel, aluminum, zinc, zinc coated iron andsteel, copper and its alloys including brass and bronze, magnesium,cadium, "taniurn, zirconium and surfaces in which one or more of thesemetals constitutes the predominant ingredient. The solutions of and themethod of this in vention are particularly satisfactory and useful whenapplied to the surface of metals in which the predominant metallicconstituent is iron, zinc or aluminum.

The solutions of this invention contain as the essential coating-formingingredients a hexavalent chromium compound, preferably chromic acid, andtertiary butyl alcohol. It has been found that the solutions should notcontain water. When water, or other high boiling organic solvent ispresent the application of heat to the cn V g 2,927,046 Patented Mar. l,1968 'filrn of the solution on the surface evaporates the tertiary butylalcohol leaving the hexavalent chromium dissolved in water or the highboiling solvent. It is necessary that the tertiary butyl alcohol bepresent in conjunction with the hexavalent chromium ion on the metalsurface during the latter stages of the heating cycle employed to dryand cause the tertiary butyl alcohol and hexavalent chromium tointer-react and become integrally hound-to the metal surface. The exactnature of the insoluble coatings of this invention is not fullyunderstood. It is known, however, that the hexavalent chromium ispresent in the coating in an insoluble or protected form in which it iseither reacted with, dissolved in or dispersed in the tertiary butylalcohol. Tests have also revealed the presence of trivalent chromium inthe coating indicating that a portion at least of the hexavalentchromium has been reduced in some manner. In addition, neither thetertiary butyl alcohol nor the, hexavalent chromium can be easily washedoff or extracted from the metal surface by aqueous or organic solvents,the metal is rendered corrosion resistant to a remarkable degree, andthe adhesion of paint, plastic coatings and other finishing materials tothe metal is markedly increased. The metal is improved in corrosionresistance to such a degree, for example, on iron, steel, zinc, copper,brass, aluminum and other metals and especially on zinc, copper andaluminum, that the unpainted coated metal can be stored in a humidatmosphere without further protection such as oiling. The resistance ofthe bare, coated metal, as shown in salt spray and humidity tests, isfrequently con.- siderably better. than comparable surfaces coated withphosphate coatings. When painted, the coated metal usually exhibitsbetter salt spray and humidity resistance and adhesion of paints thansimilar painted surfaces which have been coated first with an oxalate oraqueous chromic acid rinsed-phosphate coating. These remarkable resultsare obtained by a two-step process comprising (1) wetting the surfacewith the solution, and (2) heating to fix or insolubilize the coating,whereas phosphate and oxalate coating processes include 3 or more steps,such as (l) prolonged standing in contact with the phosphate or oxalatesolution, (2) rinsing in water, (3) dilute.

aqueous chromic acid rinse, and (4) drying.

The hexavalent chromium compound in thecoatings and in the solutions ofthis invention may be derived from chromium trioxide or chromic acidanhydride (Cl g), 0r chromic acid itself. Chromic trioxide is mostpreferred since it is appreciably soluble in tertiary butyl alcohol, isconveniently handled, and does not contain water of hydrationto bedriven off during drying and insolubilization.

The solutions of this invention should contain hexavalent chromiumequivalent to that derived from at least about 1% of chromic anhydride,CrO in the solution, wcight/ volume, that is at least about 1 gram ofCrO per ml. of the solution, the specific gravity of the solution beingassumed to be unity, for convenience. As used hereinafter'and in theappended claims proportions indicated. in percent are intended to referto percent weight/volume. In some cases slightly smaller amounts of CrOcan be utilized to form an insoluble coating. In other casesslightlylarge amounts of CrO will be required to form such a coating andboth. good corrosion resistance and good adherence for finishingmaterials. In most cases the equivalent of at least 1% of CrO in thesolution will be'suflicient to form good coatings. Still greaterproportions of hexavalent chromium of, for example, up to 10% CrO maybeemployed and proportions as high as the saturation value of theparticular hexavalent chromium compound in the tertiary butyl alcoholcan be used if desired. Usually, however, from 1% to about 4% of CrO orits equivalent, weight/ adherent thereto.

volume, will be satisfactory under commercial operating conditions.

The method of this invention is effected by the steps of applying to aclean metallic surface a solution containing as the essentialcoatingproducing ingredients tertiary butyl alcohol and hexavalentchromium, in the above indicated proportions, by immersion of the metalin the solution, by spraying the solution on the metal surface, byroller coating the surface or by any other conventionally employedmethod which is adapted to form a 1111!- form film of the solution onthe surface, and thereafter heating the coating to drive off a part ofthe tertiary butyl alcohol and cause the above described inter-reactionand bonding of the heat reaction product to the metal surface. It isunnecessary to employ any unique cleaning procedure for preparing themetal surface to be coated to receive the solutions of this invention.In general, the condition of the metallic surface does not appear to beas critical as it is in the more conventional phosphate and oxalatecoating processes. If the surface of the metal is dirty or oily, orcovered with scale or corrosion prodnote, it is desirable to remove theoil by spraying, immersion, wiping or exposing it to the vapors of asolvent, and to loosen and dissolve away other types of dirt andcorrosion with conventional acid and/ or alkaline cleaners.Insoluibilization of the coatings, or the formation of the heat reactionproduct between the tentiary butyl alcohol and hexavalent chromium,occurs at temperatures above about 200 F. and in the range of about 200F. to about 500 F. in as little as about /2 minute to as much as 3 or 4minutes, or slightly more. In no case should the coatings be so stronglyheated, or heated for such an extended period as to result inappreciable chairing of the coating.

For use as a paint base the reacted coating should be uniformlydistributed and have a weight of about to about 50 mg./sq. ft.,preferably about 10 to about 40 mg./sq. ft. For bare corrosionprotection the coating weight can satisfactorily extend as high as about200 rug/sq. it, but coatings having a weight in the range of 50-400mg./sq. ft. are usually satisfactory for this purpose. The insolublecoating thus produced imparts extremely good bare, unpainted, corrosionresistance and painted corrosion resistance to most metals and makesmost paints, plastics and finishing materials tenaciously The coatingsare effective for these purposes in their as-obtained condition and donot require further treatment.

The following examples more fully describe the solutions, method andarticles of this invention, but it is to be understood that theparticular solutions disclosed therein and the conditions of treatmentemployed are illustrative only and that the above given ranges ofcomposi; tion and conditions represent the controlling conditions ofthis invention.

Example 1 Solutions were made up by adding 1, 2 and 4% (wt/vol.) ofchromic trioxide (CrO as a stock solution in tertiary butyl alcohol toundiluted tertiary butyl alcohol. By analysis, the CrO content of theresulting solutions was found to be, respectively, 0.98%, 1.96% and3.56%, weight/volume. Panels of two difierent types of hot-dipped zinccoated steel sheet, one known as Armco Stabilized Zincgn'p and the otheras Armco Zincgri panels of ordinary cold rolled steel; and panels of analuminum alloy known as 243 were dipped in the solutions and dried in anair oven at 250 F. In every case a uniform, attractive and insolublecoating was obtained. Prior to immerson, the panels were cleaned ineither of cleaning solutions A or B. Solution A consisted of an aqueoussolution of sodium orthosilicate made up to a concentration of 48 gramsper liter. Solution B had a pH of 5.15 and consisted of 12.5 grams perliter of NaH PO and 0.5% C10 The panels were immersed in the cleaningsolutions for about 5 minutes at about F. and then rinsed in warm waterand dried. The chromic acid tertiary butyl alcohol coatings onzinc-coated steel and cold rolled steel were compared to controlsprepared by phosphate coating panels of the some stock which had beencleaned in the same fashion and, after phosphate coating, conventionallyrinsed with a dilute aqueous solution of chromic acid having a pH ofabout 2.5 to 3 and containing about 0.095 to 0.1% CrO then dried andfinally painted in the same fashion. The phosphate solution employed inthe preparation of the control panels contained zinc phosphate and minoramounts of copper, nickel, nitrite, and fluoborate ions as acceleratorsand analyzed as having a pH of 2.6 and a total acid of 19.8 to 20.1points. The aluminum panels coated with the insoluble chromate-alcoholcoatings were 7 not painted but were compared for bare corrosionresistance against bare phosphate-coated controls prepared as described.The insoluble chromate tertiary butyl alcohol coatings on steel andzinc-coated steel obtained from the solutions containing 1, 2 and 4%chromic trioxide were better than the phosphate coated, chromic acidrinsed control panels in painted salt spray resistance, painted humidityresistance, and physical adhesion of the paint. On aluminum, thecoatings of this invention were especially resistant in the barecorrosion test, the metal in some cases showing less corrosion after1176 hours exposure than the phosphate coated controls showed after only504 hours. 4

Example 2 A first solution was prepared to contain 4% CrO in tertiarybutyl alcohol, weight/volume.

A series of solutions were prepared containing 4% CrO and mixtures oftertiary butyl alcohol and a high boiling hydrocarbon solvent in theproportions of 75 tertiary butyl alcohol 25 hydrocarbon solvent, 60tertiary butyl alcohol 40 hydrocarbon solvent, 50 tertiary butyl alcohol50 hydrocarbon solvent.

The high boiling solvent employed is commercially known as Amsco solventNo. 140, which has a boiling range of C.-20'7 C., a flash point of 143F. an evaporation speed of 58 minutes and a specific gravity of .803. Aplurality of 4" x 6" cold rolled steel panels which were preliminarilymineral spirits and dry rag wiped were dipped in each of the 4 baths,removed and heated for 2 minutes at 250 F. Some of the panels werepainted with a commercial paint system designated Nubelite 3820 andothers were left unpainted. Separate painted panels, as treated in eachbath were then scratched diagonally and subjected to 20% salt spray for504 hours. The panels processed in the solutions containing thehydrocarbon solvent were in all cases complete failures after 504 hourswhereas no corrosion was detected on the panels from the bath containingonly tertiary butyl alcohol and chromic acid. Moreover the panels fromthe tertiary butyl alcohol-chromic acid bath were more adherent asdetermined both by bending and scratching tests which are conventionalin the art.

A plurality of 4" x 6" aluminum panels, aluminum 3S, were mineralspirits and dry rag wiped and dipped in the same solutions used abovewith the steel panels, at room temperature, removed and heated at 250for two minutes. These panels were then subjected to salt spraycorrosion testing for 504 hours in 20% salt fog to 552 hours in ahumidity cabinet and were also evalu ated for adhesion. The panels fromthe solution con taining 75% tertiary butyl alcohol and 25% hydrocarbonsolvent were inferior in salt spray resistance to the panels from thesolution containing only tertiary butyl alcohol and chromic acid. Thepanels from the bath containing 60 tertiary butyl alcohol and 40hydrocarbon solvent were much worse in salt-spray resistance, and thepanels from the solution containing 50 tertiary butyl alcohol and 50hydrocarbon solvent were worse than the panels from the baths containing25 and 40% hydro carbon solvent and were much poorer than the panelsobtained from the solution containing only tertiary butyl alcohol andhexavalent chromium. In humidity resistance the panels from thesolutions containing 75 tertiary butyl alcohol and 25 hydrocarbonsolvent were approximately the same as that of the panels obtained fromthe solution containing only tertiary butyl alcohol and chromic acid,but the panels containing 40% and 50% of hydrocarbon solvent wereinferior in humidity resistance to those obtained from the solutioncontaining only tertiary butyl alcohol and chromic acid. The panels fromthe solutions containing only tertiary butyl alcohol and chromic acidwere superior in adhesion to the panels obtained from each of the otherthree baths as measured both by bending and scratch tests ofconventional type.

Example 3 In this example aqueous solutions of chromic acid and tertiarybutyl alcohol were prepared containing about 0.1% (wt/vol.) of apolyoxethylene type of non-ionic dispersant known as Igepal (IgepalCA-630, an isooctylphenolpolyoxyethylene with approximately 8 to 9oxyethylene groups) and 0.01% of triethanol amine, both based on thetotal volume of solution. The dilution of the alcohol with water rangedfrom as much as 95% (i.e. 5% alcohol) to as little as 60% (i.e. 40%alcohol). In these solutions, the CRO content was about 1%, 2%, 4% and6% by weight based on solution volume. Cleaned panels of cold rolledsteel, zinc-coated steel (Armco Zincgrip), and aluminum alloys 24S and38 were dipped in the bath at room temperature and dried in an oven at250 F. At this temperature all solutions gave insoluble coatings exceptthose containing only 5% tertiary butyl alcohol and 4% to 6% chromicacid. When the dry off temperature of the latter was raised to about 350F., insoluble coatings were obtained. The properties of the coatingsprepared from aqueous solutions on steel were not as good as thoseprepared from undiluted tertiary butyl alcohol chromic trioxidesolutions in salt spray resistance, and the coatings on aluminum wereinferior in bare corrosion resistance to those obtained from thesolutions containing only tertiary butyl alcohol and chromic acid. Whileboth the bare and painted salt spray resistance, the painted humidityresistance and the physical adhesion of paint were much better than thatof the bare metal in each case, the properties were inferior to those ofthe phosphate-coated controls.

In the absence of the Igepal wetting agent the coatings were notuniformly distributed and occurred as beads scattered over the surface.

This application is a continuation-in-part of my copending application,Serial No. 519,619, filed July 1, 1955, now abandoned.

What is claimed is:

1. A composition for coating metallic surfaces which consistsessentially of tertiary butyl alcohol and chromic acid, said chromicacid being dissolved in said tertiary butyl alcohol in an amount betweenabout 1% and saturation.

2. A composition for coating metallic surfaces which consistsessentially of tertiaiy butyl alcohol and chromic acid, said chromicacid being dissolved in said tertiary butyl alcohol in an amount betweenabout 1% and about 4%.

3. A composition for coating metallic surfaces consisting of tertiarybutyl alcohol and chromic acid, said chromic acid being dissolved insaid tertiary butyl alcohol in an amount between about 1% andsaturation.

4. A composition for coating metallic surfaces consisting of tertiarybutyl alcohol and chromic acid, said chromic acid being dissolved insaid tertiary butyl alcohol in an amount between about 1% and about 4%.

5. A method of forming a coating on a metallic surface which comprisesthe steps of (l) contacting the said metallic surface with a solutionconsisting essentially of tertiary butyl alcohol and chromic acid, saidchromic acid being dissolved in said tertiary butyl alcohol in an amountbetween about 1% and saturation to form a uniform film of the saidsolution on the surface and (2) heating the said film to a temperaturein the range of about 200 F. to 500 F. for a time sufiicient to form auniformly distributed adherent coating on said surface, said coatinghaving a weight in the range of about 10 ing/sq. ft. and about 200rug/sq. ft.

6. A method of forming a coating on a metallic surface which comprisesthe steps of (l) contacting the said metallic surface with a solutionconsisting essentially of tertiary butyl alcohol and chromic acid, saidchromic acid being dissolved in said tertiary butyl alcohol in an amountbetween about 1% and 4% to form a uniform iilm of the said solution onthe surface, and (2) heating the said film to a temperature in the rangeof about 200 F. to 500 F. for a tirne sufiicient to form a uniformlydistributed adherent coating on said surface, said coating having aweight in the range of about 10 mg./sq. ft. and about 200 mg./sq. ft.

7. A metallic article having on its surface an integrally bonded coatingcomprising the reaction product of heating on said surface a uniformlydistributed film of a solution consisting essentially of tertiary butylalcohol and chromic acid, said chromic acid being present in saidsolution in an amount between about 1% and saturation, said coatinghaving a weight in the range of about 10 rug/sq. ft. to about 200mg./sq. ft.

8. A metallic article having on its surface an integrally bonded coatingcomprising the reaction product of heating on said surface a uniformlydistributed film of a solution consisting essentially of tertiary butylalcohol and chromic acid, said chromic acid being present in saidsolution in an amount between about 1% and 4%, said coating having aweight in the range of about 10 rug/sq. ft. to about 200 mg./sq. ft.

References (Iited in the file of this patent UNITED STATES PATENTS630,867 Dickey Aug. 15, 1899 1,926,766 Englund Sept. 12, 1933 2,206,064Thompson July 2, 1940 2,762,732 Somers Sept. 11, 1956

1. A COMPOSITION FOR COATING METALLIC SURFACES WHICH CONSISTSESSENTIALLY OF TERTIARY BUTYL ALCOHOL AND CHROMIC ACID, SAID CHROMICACID BEING DISSOLVED IN SAID TERTIARY BUTYL ALCOHOL IN AN AMOUNT BETWEENABOUT 1% AND SATURATION.